1. Technical Field
The present invention relates to light-emitting diode (LED) lighting and like devices in which optical emission from a blue LED excites a phosphor, causing it to emit light.
2. Description of the Related Art
Blue LEDs that emit light having an optical emission peak at 420 nm to 490 nm wavelength have been known to date. In turn, white LED lighting devices that emit pseudo-white light by causing, via optical emission from a blue LED, yellow light to luminesce from a yellow phosphor such as emits excitation light having an optical emission peak at 550 nm to 590 nm wavelength, and mixing the blue light that the blue LED emits and the yellow light that the yellow phosphor emits have been widely employed. As the yellow phosphor, yttrium aluminum garnet (YAG) phosphors are widely employed. Such white LED lighting devices are widely employed in backlights for liquid-crystal displays and in general lighting applications.
With white LED lighting devices with a blue LED being the light source and employing a YAG phosphor as a yellow fluorescent mass, as indicated in FIG. 13, in general, blue light B from a blue LED having an optical emission peak near 460 nm, and yellow light Y that is in a complementary color relationship with the blue light, luminesced from a yellow phosphor having an optical transmission peak near 580 nm, are color-mixed so as to be in a predetermined color proportion, whereby they are adjusted so as to yield white light W on a line segment YB joining the two color coordinates. Color coordination is carried out such as to adjust the vector in the blue direction and the vector in the yellow direction so as to position them on the color coordinates of the luminesced color that is the objective. FIGS. 10 through 12 graph spectra representative of pseudo-white light obtained by color-mixing blue light from the blue LED and yellow light that is its color complement that the YAG phosphor emits. In this way, as indicated in FIG. 14, white the light that is in the American National Standards Institute (ANSI) C 78.377 region, as well as the color temperature calculated by the method set forth in the appendix to JIS Z 8725, and white light that is in the xy-coordinate range of the correlate color temperature, are obtained.
A problem with the light from white LED lighting devices in which a blue LED is the light source and that employ YAG phosphors as a yellow fluorescent mass has been that because it is just blue light and yellow light color-mixed, the device's color rendering property, compared with natural light that exhibits a broad spectrum, has been inferior. “Color rendering property” is a property of a light source that, with the light source illuminating an object, is exerted in a difference from how the colors can be seen with natural light illuminating the object. The spectrum of natural light has a wide range of spectral wavelengths. On the other hand, because pseudo-white light as described above is simply light in which yellow light and blue light are color-mixed, in illuminating an object that is red or green, being a color that either is not contained in pseudo-white light or whose contained proportion is slight, the way the color appears will differ from when natural light is shone.
As white LED lighting devices in which the color rendering property of a white LED lighting device employing a blue LED and a YAG phosphor has been improved, white LED lighting devices as, for example, disclosed in below-noted Patent Document 1 and below-noted Patent Document 2 in which, in a white LED lighting device employing a blue LED and a YAG phosphor, slight quantities of a red fluorescent mass and a green fluorescent mass have been complementarily added are known. According to such methods, further color-mixing red light and green light components in a white LED lighting device employing a blue LED and a YAG phosphor enables its spectrum to approach a natural light spectrum.
As indicated in FIG. 10 to FIG. 12, in a white LED lighting device with a blue LED being the light source and employing a YAG phosphor, a large amount of blue light in the vicinity of the principal wavelength 460 nm is contained as an essential component. Reports that among visible beams of light, blue-light wavelengths—380 nm to 495 nm wavelength—can have a negative impact on the human retina have been numerous in recent years (for example, below-noted Non-Patent Document 1). Blue light is high-energy light in the visible range having wavelengths closest to ultraviolet rays, and are reputed to reach the retina without being absorbed by the cornea or the lens of the eye.
With concern over the influence of blue light reaching to the retina, eyeglasses such as disclosed, for example, in below-noted Non-Patent Document 2, in which the blue light is selectively cut, are being marketed. It is stated that when such eyeglasses are put on and a liquid-crystal display employing a white LED lighting device is viewed, in terms of numerical values based on British standard BS2724: 1987, blue light is cut by 50% or more. In addition, wavelength filters that will by covering a liquid crystal display selectively cut blue-light wavelengths are also being marketed.